| The tempered martensite embrittlement (TME) has been analyzed in terms of impact toughness, fracture behavior, and microstructures in the 4140 steel and the 4340 steel with the Ni addition. On isothermal tempering, THE was detected by the continuous decreases in impact toughness, at 200 and 250℃ for 1-400 h. On isochronal tempering for 1 h, the THE troughs with the minimum at 300℃ in the 4140 steel and at 350℃ in the 4340 steel were observed. The presence of coarse carbides at the impurity-weakened grain boundaries led to the intergranular TME. As the impact toughness decreased in the TME region, the intergranular area extended. The intergranular fracture was mostly observed at the low level near or below about 5 J. THE in the 4340 steel was presented by a slower process, as compared to the 4140 steel. This may be caused by the delay of the formation of coarse boundary-carbides and by the high intrinsic toughness with the Ni-addition, requiring the coarser boundary-carbides for the activation of brittle intergranular fracture. |
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